Several collision accidents have occurred at airports where an aircraft or vehicle has entered a runway environment which is already occupied by another aircraft that is moving at significant speed. Airborne collision protection and mitigation is provided by Traffic Collision and Avoidance System (TCAS), however the algorithms used in TCAS systems are not well suited to the airport surface operations problem; on airports, near runways, aircraft commonly operate at relatively high speeds in close proximity to other aircraft and vehicles. For example, an aircraft waiting to enter a runway is commonly stopped within a distance of the order of 100 feet from a runway that may be occupied by a landing aircraft traveling at speeds greater than 100 knots, thereby confusing TCAS algorithms. Also, on the ground at normal taxi speeds, an airplane can change its direction of travel much more rapidly than can an airborne aircraft.